Oriana Brea

Creating σ‐Holes through the Formation of Beryllium Bonds

Through the use of ab initio theoretical models based on MP2/aug-cc-pVDZ-optimized geometries and CCSD(T)/aug-cc-pVTZ and CCSD(T)/aug-c-pVDZ total energies, it has been shown that the significant electron density rearrangements that follow the formation of a beryllium bond may lead to the appearance of a σ-hole in systems that previously do not exhibit this feature, such as CH3 OF, NO2 F, NO3 F, and other fluorine-containing systems. The creation of the σ-hole is another manifestation of the bond activation-reinforcement (BAR) rule. The appearance of a σ-hole on the F atoms of CH3 OF is due to the enhancement of the electronegativity of the O atom that participates in the beryllium bond. This atom recovers part of the charge transferred to Be by polarizing the valence density of the F into the bonding region. An analysis of the electron density shows that indeed this bond becomes reinforced, but the F atom becomes more electron deficient with the appearance of the σ-hole. Importantly, similar effects are also observed even when the atom participating in the beryllium bond is not directly attached to the F atom, as in NO2 F, NO3 F, or NCF. Hence, whereas the isolated CH3 OF, NO2 F, and NO3 F are unable to yield F⋅⋅⋅Base halogen bonds, their complexes with BeX2 derivatives are able to yield such bonds. Significant cooperative effects between the new halogen bond and the beryllium bond reinforce the strength of both noncovalent interactions.

On the stability of [(uracil)2-Cu]2+ complexes in the gas phase. Different pathways for the formation of [(uracil-H)(uracil)-Cu]+ monocations

The association of uracil dimers and copper(II) has been studied through the use of B3LYP/6-311+G(3df,2p)//6-31+G(d,p) calculations. Although uracil-Cu(2+) complexes have never been experimentally detected, our results show that [(uracil)2-Cu](2+) is thermodynamically stable with regard to both the proton loss and the fragmentation into (uracil)2(+)˙ + Cu(+), although it is metastable with respect to the coulomb explosion yielding [uracil-Cu](+) + uracil(+)˙. Importantly, a proton transfer from [(uracil)2-Cu](2+) to a third neutral uracil molecule is very exothermic. This is consistent with the fact that when electrospray mass spectrometry techniques are used [(uracil-H)(uracil)-Cu](+) and uracil-H(+) monocations are detected, but not the [(uracil)2-Cu](2+) doubly charged species. In the most stable conformers of [(uracil)2-Cu](2+) the two uracil monomers are held together through the metal cation which forms a linear bridge between two carbonyl groups each belonging to a different monomer. This is at variance with what has been found for complexes involving alkaline-earth dications, such as (uracil)2Ca(2+), in which the metal dication association preserves the network of hydrogen bonds which stabilize the free (uracil)2 dimers. The formation of [(uracil-H)(uracil)-Cu](+) complexes is accompanied by the enolization of the uracil units. All possible mechanisms to reach the experimentally detected [(uracil-H)(uracil)-Cu](+) singly charged ions, either by direct association of Cu(2+) to uracil dimers and posterior deprotonation of the formed complex or through the interaction of Cu(2+) with uracil followed by its deprotonation and subsequent association with a second uracil molecule, have been investigated.

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds

High-level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y-R (Y=F, OH, NH2 ; R=CH3 , NH2 , OH, F, SiH3 , PH2 , SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX2 (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y-R bond after the formation of the beryllium bond, and the huge stabilization of the F(.) (OH(.) , NH2 (.) ) radical upon BeX2 attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.

Beryllium-based Anion sponges. Close relatives of proton sponges.

Through the use of high-level ab initio and density functional calculations it is shown that 1,8-diBeX-naphthalene (X=H, F, Cl, CN, CF3 , C(CF3 )3 ) derivatives behave as anion sponges, very much as 1,8-bis(dimethylamino)naphthalene derivatives behave as proton sponges. The electron-deficient nature of the BeX substituents, which favors strong charge transfer from the anion towards the former, results in anion affinities that are among the largest ones reported for single neutral molecules.

The Spin-Partitioned Total-Position Spread Tensor: An Application To Diatomic Molecules

The spin partition (SP) of the total-position spread (TPS) tensor is applied to the case of a few light diatomic molecules at full configuration interaction (FCI) level. It appears that the SP-TPS tensor gives informations that are complementary with respect to the corresponding spin-summed (SS) quantity. The spin-summed total position-spread tensor (SS-TPS, Λ) is defined as the second moment cumulant of the total position operator, and the SP-TPS is its partition in equal (Λαα+ββ) and different spin (Λαβ+βα) contributions. Then, the SS-TPS allows description of the molecule charge mobility, while the SP-TPS allows description of the spin delocalization. The most relevant Cartesian-component for both tensors (SS-TPS and SP-TPS) is the component along the chemical bond (Λ(∥)), and it was found that its behavior was related to the type of interaction involved. For covalent bonds the SP-TPS has a squared growth when the bond is stretched, while for ionic bonds there exists a faster-than-linear growth after the avoided-crossing between the covalent and the ionic states. Other exotic bonds, like He2 and Be2, were also considered, and a particular spin delocalization was able to describe the different character of the two weakly bonded molecules, and specially the multireference character of the wave function along the dissociative potential energy curve.

Super Strong Be-Be Bonds: A Theoretical Insight into the Electronic Structure of Be-Be Complexes with Radical Ligands

The electronic structure of complexes formed by the interaction of Be2 with radical ligands (L:Be-Be:L) has been studied by means of the high-level theoretical protocol CCSD(T)/cc-pVTZ. At this level of theory, no matter the nature of the ligand, the Be-Be bond becomes up to 300 times stronger compared to isolated Be2, indicating that these kinds of complexes are thermodynamically stable and, thus, that they could be experimentally detected. Moreover, among all of the ligands considered, the strength of the Be-Be bond for L = [CN]• was calculated to be 330 kJ·mol-1, slightly greater than the strongest up to date L = F• complex, thus setting a new mark for the strongest Be-Be bond reported so far in the literature. Wave function analysis methods explain this strong interaction as a result of the oxidation of the Be2 moiety to Be22+ due to charge transfer toward the L ligands. In this study, we have also considered F:Mg-Mg:F complexes, which show very similar properties as the ones described for the analogous F:Be-Be:F.

Ga+ Basicity and Affinity Scales Based on High‐Level Ab Initio Calculations

The structure, relative stability and bonding of complexes formed by the interaction between Ga(+) and a large set of compounds, including hydrocarbons, aromatic systems, and oxygen-, nitrogen-, fluorine and sulfur-containing Lewis bases have been investigated through the use of the high-level composite ab initio Gaussian-4 theory. This allowed us to establish rather accurate Ga(+) cation affinity (GaCA) and Ga(+) cation basicity (GaCB) scales. The bonding analysis of the complexes under scrutiny shows that, even though one of the main ingredients of the Ga(+) -base interaction is electrostatic, it exhibits a non-negligible covalent character triggered by the presence of the low-lying empty 4p orbital of Ga(+) , which favors a charge donation from occupied orbitals of the base to the metal ion. This partial covalent character, also observed in AlCA scales, is behind the dissimilarities observed when GaCA are compared with Li(+) cation affinities, where these covalent contributions are practically nonexistent. Quite unexpectedly, there are some dissimilarities between several Ga(+) -complexes and the corresponding Al(+) -analogues, mainly affecting the relative stability of π-complexes involving aromatic compounds.